In the past, pipes for below-ground installation have been covered with various materials such as fusion-bonded epoxy resin, hot applied coal tar or asphalt, polyethylene, glass cloth, asbestos felt, and cold-applied plastic tapes. Prior to placement of many outer wraps on the pipe, the pipe is coated with a suitable waterproofing material, such as coal tar pitch, asphalt, or wax, and thereafter wrapping the coated pipe with felt, plastic film, or glass cloth. Since bitumens and wax are ordinarily solids at temperatures less than 150.degree. F, they are heated to a readily flowable consistency and usually applied to the pipe by flooding the hot wax or bitumen on to the pipe as it moves horizontally while rotating. The pipe rotation can be mechanically set to rotate one full revolution as the pipe moves horizontally at a rate such that the distance traveled during a single revolution of the pipe is slightly less than the width of the wrapping material, thus causing each adjacent width of the latter to overlap on each revolution. Thus, if a 9-inch-wide wrap is applied, the pipe moves horizontally about 81/2 inches for each full revolution, providing an overlap of about 1/2 inch.
Once the thus coated and wrapped pipe is buried in the ground, it generally is further protected by impressing a current thereon to prevent corrosion where defects or holidays exist in the coating. These gaps or holidays in the coating are caused by imperfect application of the coating or damage to the coating and wrapping material during installation. One method of protecting such pipe in place involves the use of a suitable sacrificial anode or rectifier-operated anode. Such systems reverse electron flow from the exposed portion of the pipe to the latter from the anode.
Prior to backfilling, the coatings and wrappings are tested for holidays with a high-voltage detector. This generally is set between 7,000 to 20,000 volts and operated at low amperage to prevent breakdown of the coating. The voltage varies according to coating thickness. In the event a holiday or void exists in the coating, the voltage has sufficient potential to develop a spark between the charged detector ring on the outside of the pipe and the pipe surface. The voltage has to be set at a level that will develop a spark which will be at least the applied coating thickness.
Pipe coverings or outer wraps, such as asbestos felt, to aid in inhibiting corrosion of buried pipe, have been used for more than 50 years. More recently, glass cloth and plastic film have been employed. The felt-wrapped coating has the disadvantage that the soil becomes firmly bonded to it. Movement of the pipe or settling of the backfill splits and buckles the protective covering since the hot-applied bitumens are more firmly bonded to the inner surface of the felt than to the pipe. This has caused extensive failure of felt on the top side due to backfill settlement. Felt-wrapped pipe has also failed badly in areas where excessive wet and dry cycles occur, particularly in the Gulf Coast region, and in irrigated areas where the ground is alternately very wet and very dry.
In the case of glass cloth, the round glass fibers cold-flow into the enamel coating causing the coating to extrude outwardly through the cloth, although it does prevent mechanical damage to the coating. Tests and field observations conducted by pipeline companies on tar coatings wrapped with glass cloth show the flow of the tar through this type of wrapper. Such flowthrough is due to the round shape of the glass fibers which cut into the soil-pressured tar and extrude it outside of the glass fibers. Therefore, round glass fibers do not function effectively as a reinforcing material unless covered by a plastic film.
Hot-applied coatings wrapped with polyethylene film are not affected by soil attachment and the underlying coating is not split or moved substantially by the backfill. However, in the event of a holiday in the coating under the film, the latter can prevent the holiday-inspection equipment from functioning properly as the dielectric of the film itself prevents the spark from traveling through the film into the pipe through the holiday. Also, the film creates a high-dielectric shield that can interfere with the current flow from the impressed cathodic protection anode. This is of particular concern in the long-term operation of the pipeline. Efficient cathodic protection requires the current to flow uniformly to the entire pipe surface and not to be impeded by a high-dielectric wrap covering the pipe and possibly a holiday in the coating. Such shielded holidays are not adequately protected by the impressed current.